Magnetic resonance imaging apparatus and magnetic resonance imaging method

ABSTRACT

An MRI apparatus includes an imaging signal acquisition unit, a motion signal acquisition unit, a motion amount determination unit, a motion correction unit and an image reconstruction unit. The imaging signal acquisition unit acquires MR signals as imaging signals. The motion signal acquisition unit repetitively acquires MR signals having PE amount less than that of the imaging signals as motion signals. The motion amount determination unit obtains a motion amount using the motion signals. The motion correction unit performs correction processing of the imaging signals in accordance with the motion amount. The image reconstruction unit reconstructs an image using the imaging signals after the correction processing.

CROSS REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. patent application Ser. No.11/699,325 filed Jan. 30, 2007 (now issued U.S. Pat. No. 7,365,543),which claimed priority from Japanese applications No. 2006-028286 filedFeb. 6, 2006, and No. 2006-332465 filed Dec. 8, 2006, the entiredisclosures of which priority applications are incorporated herein byreference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a magnetic resonance imaging apparatusand a magnetic resonance imaging method which generate gradient magneticfields on an imaging area formed in a static magnetic field, resonatesnuclear spins in the object set in the imaging area magnetically bytransmitting radio frequency signals and reconstructs an image of theobject by using nuclear magnetic resonance signals generated due to anexcitation, and more particularly, to a magnetic resonance imagingapparatus and a magnetic resonance imaging method in which influence ofmotion of the object on an image is reduced by correction.

2. Description of the Related Art

An MRI (Magnetic Resonance Imaging) apparatus is an apparatus whichgenerates gradient magnetic fields by gradient coils on an imaging areaof an object set in a cylindrical static field magnet for producing astatic magnetic field, resonates nuclear spins in the objectmagnetically by transmitting RF (Radio Frequency) signals from an RFcoil and reconstructs an image of the object by using NMR (NuclearMagnetic Resonance) signals generated due to an excitation.

Imaging of the heart under a magnetic resonance imaging method with useof the magnetic resonance imaging apparatus has been further developedin recent years. Typical applications of the imaging of the heartinclude high resolution imaging for the blood vessel figure of thecoronary artery. In the high resolution imaging for the coronary artery,an influence of a breathing motion on an image needs to be reduced asmuch as possible.

One of measurements for suppressing the influence of the breathingmotion is breath-holding imaging in which the imaging is performed whenthe breath is held. However, in the breath-holding imaging, the imagingis only performed during a breath-holding period and there is alimitation on the resolution. In addition, there is a fear in the degreeof stability of the breath-holding.

Another method of suppressing the influence of the breathing motionrelies on a technique of using synchronous imaging as well under a freebreath condition. A synchronous signal used in the synchronous imagingcan be obtained by an expansion and contraction sensor or a pressuresensor arranged around the abdominal part of an object. However, thereis a problem in that this synchronous imaging method with use of thesynchronous signal obtained by the expansion and contraction sensor orthe pressure sensor has insufficient accuracy.

In view of the above, another method instead of the synchronous imagingmethod with use of the expansion and contraction sensor or the pressuresensor, there are proposed a synchronous imaging method in which theposition of the diaphragm detected on the basis of NMR signals from thediaphragm is used as a synchronous signal (for example, Liu et al.,Magnetic Resonance In Medicine, 30, pages 507-511, (1993)) and animaging method for reflecting positional information of a moving imagingarea on a control for collecting the NMR signals for imaging to finelyadjust an excited slice position.

FIG. 22 is a diagram explaining an area for acquiring NMR signals fordetecting motion of a diaphragms in a conventional magnetic resonanceimaging apparatus. FIG. 23 is a diagram showing a conventional pulsesequence defining an imaging condition to acquire data for detectingmotion and that for imaging shown in FIG. 22.

In a synchronous imaging method using the position of the diaphragm asthe synchronous signal, as shown in the solid line frame of FIG. 22,other than a data collection area for imaging including the heart, anarea on the cylinder as shown in the dotted line frame including thediaphragm is set as the data collection area for the motion detection.

Then, the imaging is executed in accordance with the pulse sequenceshown in FIG. 23. In the general pulse sequence, a pre-pulse such as afat suppression pulse is used in combination in many times, and prior tothe sequence for imaging, a sequence for applying the pre-pulse is set.Then, prior to the sequence for the pre-pulse, a sequence for the motiondetection is set.

In addition, a sequence for applying dummy shots (which is also referredto as stabilization shots) for acquiring data necessary forpost-processing on data or information which is required in collectingdata for imaging is set at the start of the data collection for imaging.Normally, the slice direction of the dummy shot is set to an axialcross-section for the purpose of realizing the stabilization of spinningsimilarly to the data collection for imaging.

Then, on the basis of the sequence for the motion detection, the datacollection area for the motion detection including the diaphragm isexcited in a particular condition different from the exciting method forthe imaging area. Furthermore, the data for the motion detection isacquired from the collection area including the diaphragm and a signalcalled navigator is generated. Next, the position of the diaphragm isdetected from the navigator signal, and a control method for hardware atthe time of imaging is determined and adoption judgment as to the datacollection for imaging is conducted in accordance with the amount ofchange in the position of the diaphragm. In addition, the amount of thebreathing-related shift on the heart necessary for the motion correctionon the data for imaging is calculated by multiplying the amount of shiftof the diaphragm by a given ratio.

Such a navigator method in which the sequence for the motion detectionwhich is different from the sequence for imaging is used for collectingthe navigator signal to acquire the synchronous signal is applied tovarious technologies.

However, the navigator method for collecting the navigator signal undera condition different from the data collection condition for imaging hastwo major problems.

The first problem resides in that the amount of breath-related shift ofthe target area of the imaging (the heart) and the amount of shift ofthe area to be observed by the navigator signal (the diaphragm) haverelevance but are not completely the same to each other. Therefore, theamount of shift of the heart is estimated from the amount of shift ofthe diaphragm, which is a cause of decreasing the accuracy. In addition,a ratio between the amount of breath-related shift of the heart and theamount of shift of the diaphragm varies between individuals and changesdepending on the state of breathing even in the same object, and it istherefore difficult to obtain a stable image.

The second problem resides in that timing for detecting the signal fromthe diaphragm is largely different from timing for detecting the signalfor imaging. It is necessary to continuously perform the application ofthe pre-pulse and the data collection for imaging to be executed afterapplication of the pre-pulse. Therefore, the timing for collecting thenavigator signal on the basis of the sequence for collecting thenavigator signal (navigator sequence) disadvantageously needs to beseparated in view of time from timing for collecting the data on thebasis of the sequence for imaging. Then, the shift of the timing forcollecting the navigator signal from the timing for the data collectionfor imaging becomes a cause of decreasing the accuracy in a case where abreath period of the object is relatively short.

On the other hand, as another method for detecting the motion of thediaphragm, a method of generating a navigator signal even during datacollection for imaging by using a pulse sequence for imaging is proposed(for example, Ehman, Felmee, Radiology, 173, pages 255-263, (1989)).

This technique for generating the navigator signal during the datacollection for imaging includes generating a plurality of echo signalsin a spin echo sequence and utilizing one of the thus generated echosignals as the navigator signal. According to this technique, the motioninformation in the readout direction and the phase encode direction canbe observed. Then, this technique has a merit of collecting thenavigator signal and the signal for imaging substantially at the sametiming as well as a merit of observing the motion of the same area asthe imaging target, that is, the motion of the heart.

However, the conventional technology for generating the navigator signalduring the data collection for imaging suffers a problem of difficultyin observing the motion at a practically sufficient accuracy. Thisaccuracy deficient problem arises because the navigator signal is dataobtained by projecting data from the object in a particular direction.That is, the navigator signal is data obtained by superposing data fromthe area which is not in motion in actuality onto data from the areawhich is in motion as being the projection data in the particulardirection. Thus, the navigator signal is under influence of the areathat is not in motion.

For example, in the thoraco-abdominal area, fats on the body surface,the chest wall, muscles of the back, and the like are not in motion.However, these unmoved areas are closer to the reception coil thanmoving areas such as the liver, and accordingly the signal intensityfrom the unmoved area is relatively larger than the signal intensityfrom the moving area. Therefore, the accuracy in detection of the motionis easily influenced by the unmoved area, and particularly in a case ofimaging requiring a high accuracy such as high resolution imaging, theinfluence of the unmoved area on the motion detection accuracy becomes aproblem.

SUMMARY OF THE INVENTION

The present invention has been made in light of the conventionalsituations, and it is an object of the present invention to provide amagnetic resonance imaging apparatus and a magnetic resonance imagingmethod which make it possible to obtain an image with highly accuratemotion correction by acquiring motion signals sufficient for observingmotion amount directly from a part which is an imaging target and nearlysimultaneously with data acquisition for imaging.

Furthermore, it is another object of the present invention to provide amagnetic resonance imaging apparatus and a magnetic resonance imagingmethod which make it possible to obtain an image with highly accuratemotion correction by reducing influence of parts which do not move.

The present invention provides a magnetic resonance imaging apparatuscomprising: an imaging signal acquisition unit configured to acquiremagnetic resonance signals from an object as imaging signals; a motionsignal acquisition unit configured to repetitively acquire magneticresonance signals having phase encode amount less than that of theimaging signals as motion signals; a motion amount determination unitconfigured to obtain a motion amount using the motion signals; a motioncorrection unit configured to perform correction processing of theimaging signals in accordance with the motion amount; and an imagereconstruction unit configured to reconstruct an image using the imagingsignals after the correction processing, in an aspect to achieve theobject.

The present invention also provides a magnetic resonance imagingapparatus comprising: an imaging signal acquisition unit configured toacquire magnetic resonance signals from an object as imaging signals; amotion signal acquisition unit configured to obtain a part of theimaging signals and magnetic resonance signals having phase encodeamount less than that of the imaging signals as motion signals; a motionamount determination unit configured to obtain a motion amount using themotion signals; a motion correction unit configured to performcorrection processing of the imaging signals in accordance with themotion amount; and an image reconstruction unit configured toreconstruct an image using the imaging signals after the correctionprocessing, in an aspect to achieve the object.

The present invention also provides a magnetic resonance imagingapparatus comprising: an imaging signal acquisition unit configured toacquire magnetic resonance signals from an object as imaging signals; amotion signal acquisition unit configured to repetitively acquiremagnetic resonance signals having phase encode amount less than that ofthe imaging signals as motion signals; a motion amount determinationunit configured to obtain a motion amount using the motion signals; aselection unit configured to select imaging signals within a specificrange in accordance with the motion amount; and an image reconstructionunit configured to reconstruct an image using the imaging signals withinthe specific range, in an aspect to achieve the object.

The present invention also provides a magnetic resonance imagingapparatus further comprising a motion reflected component acquisitionunit configured to acquire signal component reflecting a motion from themotion signals, wherein said motion amount determine unit is configuredto obtain the motion amount from the signal component reflecting themotion, in an aspect to achieve the object.

The present invention also provides a magnetic resonance imaging methodcomprising steps of: acquiring magnetic resonance signals from an objectas imaging signals; repetitively acquiring magnetic resonance signalshaving phase encode amount less than that of the imaging signals asmotion signals; obtaining a motion amount using the motion signals;performing correction processing of the imaging signals in accordancewith the motion amount; and reconstructing an image using the imagingsignals after the correction processing, in an aspect to achieve theobject.

The present invention also provides a magnetic resonance imaging methodcomprising steps of: acquiring magnetic resonance signals from an objectas imaging signals; obtaining a part of the imaging signals and magneticresonance signals having phase encode amount less than that of theimaging signals as motion signals; obtaining a motion amount using themotion signals; performing correction processing of the imaging signalsin accordance with the motion amount; and reconstructing an image usingthe imaging signals after the correction processing, in an aspect toachieve the object.

The present invention also provides a magnetic resonance imaging methodcomprising steps of: acquiring magnetic resonance signals from an objectas imaging signals; repetitively acquiring magnetic resonance signalshaving phase encode amount less than that of the imaging signals asmotion signals; obtaining a motion amount using the motion signals;selecting imaging signals within a specific range in accordance with themotion amount; and reconstructing an image using the imaging signalswithin the specific range, in an aspect to achieve the object.

The magnetic resonance imaging apparatus and the magnetic resonanceimaging method as described above make it possible to obtain an imagewith highly accurate motion correction by acquiring motion signalssufficient for observing motion amount directly from a part which is animaging target and nearly simultaneously with data acquisition forimaging.

Furthermore, it is possible to obtain an image with highly accuratemotion correction by reducing influence of parts which do not move.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:

FIG. 1 is a block diagram showing a magnetic resonance imaging apparatusaccording to a first embodiment of the present invention;

FIG. 2 is a functional block diagram of the computer shown in FIG. 1;

FIG. 3 is a diagram showing an example of imaging area set by theimaging condition setting unit shown in FIG. 2;

FIG. 4 is a diagram showing an example of pulse sequence produced forimaging the imaging area including the heart shown in FIG. 3;

FIG. 5 is a diagram showing a specific example of the pulse sequenceshown in FIG. 4;

FIG. 6 is a diagram showing an example of imaging condition settingscreen displayed on the display unit shown in FIG. 1;

FIG. 7 is a diagram showing a specific example of pulse sequence set forreducing influence of vibration of magnetization occurred due to a dummyshot and magnetization to data acquisition for imaging by the imagingcondition setting unit shown in FIG. 2;

FIG. 8 is a diagram showing an example of motion signals arranged in thek-space in the motion detection signal acquiring unit shown in FIG. 2;

FIG. 9 is a diagram showing an example of imaging signals arranged inthe k-space in the imaging signal acquiring unit shown in FIG. 2;

FIG. 10 is a diagram showing an area of the motion signals separated bythe motion signal component separating unit shown in FIG. 2;

FIG. 11 is a diagram showing an example of profile corresponding to thearea near the heart shown in FIG. 10;

FIG. 12 is a diagram explaining a method for determining a motion amountby the motion amount determine unit shown in FIG. 2;

FIG. 13 is a diagram explaining a method for correcting imaging signalsby the motion correcting unit shown in FIG. 2;

FIG. 14 is a flowchart showing a procedure for imaging a vascular imageof the heart of the object with the magnetic resonance imaging apparatusshown in FIG. 1;

FIG. 15 is a diagram explaining a modification example of method forarranging motion signals in the k-space formed in the motion detectionsignal acquiring unit shown in FIG. 2;

FIG. 16 is a flowchart explaining a modification example of method forseparating motion signals by the motion signal component separating unitshown in FIG. 2;

FIG. 17 is a diagram showing an example of area formed by the imagereconstruction processing shown in FIG. 16;

FIG. 18 is a conceptual diagram of the motion part mask shown in FIG.16;

FIG. 19 is a block diagram showing a magnetic resonance imagingapparatus according to a second embodiment of the present invention;

FIG. 20 is a diagram showing an example of pulse sequence produced bythe imaging condition setting unit of the magnetic resonance imagingapparatus shown in FIG. 19;

FIG. 21 is a diagram showing a slab position excited by the pulsesequence shown in FIG. 20;

FIG. 22 is a diagram explaining an area for acquiring NMR signals fordetecting motion of a diaphragma in a conventional magnetic resonanceimaging apparatus; and

FIG. 23 is a diagram showing a conventional pulse sequence defining animaging condition to acquire data for detecting motion and that forimaging shown in FIG. 22.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

A magnetic resonance imaging apparatus and a magnetic resonance imagingmethod according to embodiments of the present invention will bedescribed with reference to the accompanying drawings.

FIG. 1 is a block diagram showing a magnetic resonance imaging apparatusaccording to a first embodiment of the present invention.

A magnetic resonance imaging apparatus 20 shown in FIG. 1 includes astatic field magnet 21 for generating a static magnetic field, a shimcoil 22 arranged inside the static field magnet 21 which iscylinder-shaped, a gradient coil unit 23 and a RF coil 24. The staticfield magnet 21, the shim coil 22, the gradient coil unit 23 and the RFcoil 24 are built in a gantry (not shown).

The magnetic resonance imaging apparatus 20 also includes a controlsystem 25. The control system 25 includes a static magnetic field powersupply 26, a gradient power supply 27, a shim coil power supply 28, atransmitter 29, a receiver 30, a sequence controller 31 and a computer32. The gradient power supply 27 of the control system 25 includes anX-axis gradient power supply 27 x, a Y-axis gradient power supply 27 yand a Z-axis gradient power supply 27 z. The computer 32 includes aninput device 33, a display unit 34, an operation unit 35 and a storageunit 36.

The static field magnet 21 communicates with the static magnetic fieldpower supply 26. The static magnetic field power supply 26 supplieselectric current to the static field magnet 21 to get the function togenerate a static magnetic field in an imaging region. The static fieldmagnet 21 includes a superconductivity coil in many cases. The staticfield magnet 21 gets current from the static magnetic field power supply26 which communicates with the static field magnet 21 at excitation.However, once excitation has been made, the static field magnet 21 isusually isolated from the static magnetic field power supply 26. Thestatic field magnet 21 may include a permanent magnet which makes thestatic magnetic field power supply 26 unnecessary.

The static field magnet 21 has the cylinder-shaped shim coil 22coaxially inside itself. The shim coil 22 communicates with the shimcoil power supply 28. The shim coil power supply 28 supplies current tothe shim coil 22 so that the static magnetic field becomes uniform.

The gradient coil unit 23 includes an X-axis gradient coil unit 23 x, aY-axis gradient coil unit 23 y and a Z-axis gradient coil unit 23 z.Each of the X-axis gradient coil unit 23 x, the Y-axis gradient coilunit 23 y and the Z-axis gradient coil unit 23 z which iscylinder-shaped is arranged inside the static field magnet 21. Thegradient coil unit 23 has also a bed 37 in the area formed inside itwhich is an imaging area. The bed 37 supports an object P. Around thebed 37 or the object P, the RF coil 24 may be arranged instead of beingbuilt in the gantry.

The gradient coil unit 23 communicates with the gradient power supply27. The X-axis gradient coil unit 23 x, the Y-axis gradient coil unit 23y and the Z-axis gradient coil unit 23 z of the gradient coil unit 23communicate with the X-axis gradient power supply 27 x, the Y-axisgradient power supply 27 y and the Z-axis gradient power supply 27 z ofthe gradient power supply 27 respectively.

The X-axis gradient power supply 27 x, the Y-axis gradient power supply27 y and the Z-axis gradient power supply 27 z supply currents to theX-axis gradient coil unit 23 x, the Y-axis gradient coil unit 23 y andthe Z-axis gradient coil unit 23 z respectively so as to generategradient magnetic fields Gx, Gy and Gz in the X, Y and Z directions inthe imaging area.

The RF coil 24 communicates with the transmitter 29 and the receiver 30.The RF coil 24 has a function to transmit a RF signal given from thetransmitter 29 to the object P and receive a NMR signal generated due toan nuclear spin inside the object P which is excited by the RF signal togive to the receiver 30.

The sequence controller 31 of the control system 25 communicates withthe gradient power supply 27, the transmitter 29 and the receiver 30.The sequence controller 31 has a function to storage sequenceinformation describing control information needed in order to make thegradient power supply 27, the transmitter 29 and the receiver 30 driveand generate gradient magnetic fields Gx, Gy and Gz in the X, Y and Zdirections and a RF signal by driving the gradient power supply 27, thetransmitter 29 and the receiver 30 according to a predetermined sequencestored. The control information above-described includes motion controlinformation, such as intensity, impression period and impression timingof the pulse electric current which should be impressed to the gradientpower supply 27

The sequence controller 31 is also configured to give raw data to thecomputer 32. The raw data is complex number data obtained through thedetection of a NMR signal and A/D conversion to the NMR signal detectedin the receiver 30.

The transmitter 29 has a function to give a RF signal to the RF coil 24in accordance with control information provided from the sequencecontroller 31. The receiver 30 has a function to generate raw data whichis digitized complex number data by detecting a NMR signal given fromthe RF coil 24 and performing predetermined signal processing and A/Dconverting to the NMR signal detected. The receiver 30 also has afunction to give the generated raw data to the sequence controller 31.

In addition, an ECG (electro cardiogram) unit 38 for acquiring an ECGsignal of the object P is arranged in the vicinity of the object P. TheECG signal detected by the ECG unit 38 is outputted to the sequencecontroller 31. Thus, the sequence controller 31 is configured to sendcontrol signals synchronized with the ECG signal to the gradient powersupply 27, the transmitter 29 and the receiver 30.

The computer 32 gets various functions by the operation unit 35executing some programs stored in the storage unit 36 of the computer32. The computer 32 may include some specific circuits instead of usingsome of the programs.

FIG. 2 is a functional block diagram of the computer 32 shown in FIG. 1.

The computer 32 functions as an imaging condition setting unit 40, asequence controller control unit 41, a motion detection signal acquiringunit 42, an imaging signal acquiring unit 43, a motion signal componentseparating unit 44, a motion amount determine unit 45, a motioncorrecting unit 46 and an image reconstructing unit 47 by program asshown in FIG. 2. Thus, these elements give a function to acquire an NMRsignal for detecting motion as well as that for imaging an object P andreconstruct an image with motion correction based on motion informationdetected from NMR signals for detecting motion to the magnetic resonanceimaging apparatus 20.

The imaging condition setting unit 40 has a function to provide thedisplay unit 34 with image information to display an imaging conditionsetting screen for setting an imaging condition on the display unit 34and also a function of generating a pulse sequence as the imagingcondition on the basis of information from the input unit 33. For thedisplay of the imaging condition setting screen and the input of theinformation, GUI (Graphical User Interface) technique can be used. Inaddition, the imaging condition setting unit 40 is configured to supplythe thus generated pulse sequence to the sequence controller controlunit 41.

FIG. 3 is a diagram showing an example of imaging area set by theimaging condition setting unit 40 shown in FIG. 2.

As shown in FIG. 3, an area including the heart is set as the collectionarea for the NMR signals for the imaging (imaging area), for example,and it is possible to set an imaging condition for imaging the bloodimage of the heart. In addition, an area for collecting the NMR signals(motion signals) for detecting the motion of the heart is also set asthe area including the heart. FIG. 3 shows an example in which thecollection area for the NMR signals for the imaging and the collectionarea for the motion signals are set identical to each other, that is, anexample in which the NMR signals for the imaging and the motion signalsis collected from the same excitation area.

It should be noted that the collection area for the NMR signals for theimaging can be arbitrarily set and the collection area for the motionsignals is set as the area where the motion signals can be collectedfrom body parts and organs that are the motion detection targetdirectly.

FIG. 4 is a diagram showing an example of pulse sequence produced forimaging the imaging area including the heart shown in FIG. 3.

As shown in FIG. 4, the pulse sequence generated by the imagingcondition setting unit 40 is structured to provide a sequence forapplying a pre-pulse such as a fat suppression pulse prior to thesequence of the data collection for the imaging. In addition, a sequencefor applying dummy shots for acquiring data required for apost-processing on information and data necessary for the collection ofthe data for the imaging is produced. The sequence for the dummy shotsis set close to the sequence for the imaging in view of time. In FIG. 4,the sequence for the dummy shots is set at the start of the datacollection for the imaging. It should be noted that the sequence isproduced such that the area to be applied with the dummy shots becomesthe area where the motion signals can be collected directly from theorgan that is the motion detection target.

The slice direction of the dummy shots (cross-sectional direction of theexcitation slab) can be set as an arbitral surface such as an axialsurface, a sagittal surface, a coronal surface, and another obliquecross-section, and if the slice direction is set to a direction in whichthe motion is large, it is possible to detect the motion with furthersmaller amount of data at an improved accuracy. Therefore, it ispossible to reduce the data collection time for the motion signals.Thus, a readout (RO) direction (frequency encode direction) is desirablyset to a body axis direction of the object P.

FIG. 5 is a diagram showing a specific example of the pulse sequenceshown in FIG. 4.

In FIG. 5, RF denotes a radio frequency (RF) signal, Gss denotes agradient magnetic field pulse for slice selection, that is, in a sliceencode (SE) direction, Gpe denotes a gradient magnetic field pulse in aphase encode (PE) direction, Gro denotes a gradient magnetic field pulsein a readout direction, ADC denotes a digitalized reception signal(ACQ), and ECG denotes an ECG signal. It should be noted that the encodedirection herein refers to two axes at right angles to the readoutdirection.

As shown in FIG. 5, the pulse sequence is structured, for example, by apart A, a part B and a part C. The part B is the sequence for the datasignals for the imaging, and the part A and the part C are the sequencefor the detection of the motion signals. Then, the slice direction inthe dummy shots for collecting the motion signals and the slicedirection for the data collection for the imaging are both set to thesagittal surface including the heart. It should be noted that only oneof the part A and the part C may be set to the sequence for thedetection of the motion signals. In addition, the pulse sequence may beset such that only one of or both of the part A and the part C arerepeatedly performed.

That is, the motion signals can be corrected before or after thecollection of the data signals for the imaging. It should be noted thatif the motion signals are collected both before and after the collectionof the data signals for the imaging, the number of time phases forcollecting the motion signals can be increased. Then, the motion signaldata changing over time through an interpolating process such as theinterpolation or the extrapolation can be obtained. As a result, ahigher accuracy of the motion signal data can be achieved.

The part A, the part B and the part C are all a sequence based on animaging method called SSFP (Steady-State Free Precession), for example.According to SSFP, the RF signals are applied at an extremely shortrepetition time (TR) and the data collection can be executed. Thus, itcan be said that a time difference between the part A and the part B anda time difference between the part B and the part C are sufficientlysmall. That is, if the data collection for the imaging and thecollection of the motion signals are performed with a sequence in whichTR is a small, it can be considered that the data collection timing forthe imaging and the collection timing of the motion signals are atsubstantially the same time. Thus, it can be considered the motionduring the data collection for the imaging and the motion during thecollection of the motion signals are sufficiently the same.

The part A, the part B and the part C are sequences for theelectrocardiograph synchronous imaging to be repeatedly executed insynchronous with the ECG signals acquired by the ECG unit 38. The part Bthat is the sequence for the imaging collects a three dimensionalimaging signals ACQ (B) by the readout pulse while sequentially changinga phase encode pulse PE (B), in other words, a sequence for the segmentimaging. Thus, in one sequence for the imaging during one heart beat,partial data for the imaging is obtained. Then, the data collection forthe imaging is repeatedly executed over a plurality of heart beats, andit is thus possible to collect all the data necessary for the imaging.That is, the data for the imaging is collected by performing the segmentimaging by plural times over the plurality of heart beats.

On the other hand, the part A and the part C that are the sequences forthe detection of the motion signals respectively collect motiondetection signals ACQ (A) and ACQ (C) by the readout pulses whilesequentially changing the phase encode pulses PE (A) and PE (C). Thecollection of the motion signals on the basis of the sequence for thedetection of the motion signals (the part A and the part C in theexample of the sequence shown in FIG. 5) is executed each time thesegment imaging on the basis of the sequence for the imaging proceeds,that is, at every heart beat. Thus, all the motion signals of thenecessary amount of encoding are collected each time by thecorresponding sequence for the detection of the motion signals at everyheart beat. Therefore, the motion signals from the same area areacquired over plural times at different timings intermittently. Thus,the collected motion signals contain more accurate information on thebreathing motion. Accordingly, it is possible to find out the amount ofmotion from the motion signals obtained from the same area at differenttimings more precisely.

In addition, in the sequence for the detection of the motion signals,the amount of phase encode (the number of steps of the phase encode) isset smaller than the sequence for the imaging so that the necessarymotion signals can be selectively collected at a shorter period of time.It should be noted that in the amount of phase encode in the sequencefor the detection of the motion signals is not zero. Furthermore, thesequence is set such that information that is the original collectiontarget of the dummy shots can be obtained under the imaging condition.

For example, in the dummy shot, a sequence is set so that a 45° RF pulseis applied and accordingly a signal can be obtained at a certaincontrast. Then the sequence for the imaging is set so that a 90° RFpulse is applied and accordingly the data collection for the imaging isperformed at a contrast determined on the basis of the contrast of thesignals collected in the dummy shot.

In addition, the encode directions in the sequence for the detection ofthe motion signals are not limited to biaxial directions of the sliceencode direction and the phase encode direction, and the encodedirection may be a single-axis direction of one of the slice encodedirection and the phase encode direction.

Then, the imaging condition setting unit 40 is configured to display theimaging condition setting screen for setting such an imaging conditionon the display unit 34 so that the user can generate the pulse sequenceas the imaging condition through the input unit 33.

FIG. 6 is a diagram showing an example of imaging condition settingscreen displayed on the display unit 34 shown in FIG. 1.

As shown in the upper part of the imaging condition setting screen inFIG. 6, in the dummy shots of the SSFP sequence, a button for switchingOn/Off of a function for collecting the motion signals and executing themotion correction is displayed. When this button is clicked by way ofthe operation of the input unit 33 such as a mouse, the motioncorrection function based on the motion signals collected in the dummyshots is put into an on state.

Furthermore, the number of the phase encodes (PE Matrix), the number ofslice encodes (SE Matrix) and the number of dummy shots (Num of DummyShot) in the dummy shots can be arbitrarily set with the operation suchas scrolling of a scroll bar.

Moreover, a Linear (SI) button for specifying a motion model to be usedfor the motion correction is provided at the lower part of the imagingcondition setting screen. When the Linear (SI) button is clicked, acorrection mode is selected for executing the motion correction on thebasis of the motion model in which it is assumed that the area servingas the detection target of the amount of motion is linearly deformed dueto the motion. Then, a zero-order coefficient and a first order oflinear expression representing the linear motion model can bearbitrarily set through the operation such as scrolling of the scrollbar. The detail of the motion model will be described later.

Incidentally, the phase encode pulses included in the dummy shots fordetecting the motion signals generate an eddy current. The phase encodepulses are applied by plural times intermittently with differentsurfaces and polarities in usual cases. Therefore, vibration of themagnetization is generated due to the thus generated eddy current.Therefore, there is a fear that the vibration of the magnetizationgenerated due to the dummy shots gives an influence on the datacollection for the imaging and a ghost may occur on a reconstructedimage.

In view of the above, by setting such a pulse sequence that thevibration of the magnetization generated due to the dummy shots isreduced and that the influence of the vibration of the magnetization isreduced as much as possible with respect to the data collection for theimaging, the occurrence of a ghost can be suppressed. For that reason,it is desired to set such a sequence that the vibration of themagnetization generated due to the dummy shots is gradually reduced. Inaddition, it is also desired to set such a sequence that the amount ofstep (difference in pulse intensities) between the last phase encodepulse in the dummy shots and the following first phase encode pulse forthe imaging is as small as possible.

FIG. 7 is a diagram showing a specific example of pulse sequence set forreducing influence of vibration of magnetization occurred due to a dummyshot and magnetization to data acquisition for imaging by the imagingcondition setting unit 40 shown in FIG. 2.

As shown in FIG. 7, a description will be given of a case where thesequence for the imaging is set following the sequence for the dummyshot for collecting the motion signals. It should be noted that RFdenotes an RF signal and Gpe denotes a phase encode pulse.

First of all, the step direction (application direction) of the phaseencode pulse for detecting the motion signals is set identical to thestep direction of the phase encode pulse for the imaging. Then, thesequence for the dummy shots is set so that the phase encode pulse fordetecting the motion signals alternately changes its polarity over aperiod of time and gradually reduces its pulse intensity. As a result,the vibration of the magnetization generated due to the dummy shots canbe smoothly reduced.

In addition, the sequence for the imaging is set such that the polarityof the last phase encode pulse in the dummy shots is the same as thepolarity of the first phase encode pulse for imaging and the amount ofstep (difference between the pulse intensities Gpe) of the phase encodepulses is reduced as much as possible. As a result, the influence of thevibration of the magnetization generated due to the dummy shots on thedata collection for the imaging can be suppressed. In addition, in thedata collection for the imaging as well, in order to smooth thevibration of the magnetization, the sequence is preferably set such thatthe pulse intensity is gradually increased while the polarities arealternately changed over the elapse of time.

Therefore, such a pulse sequence is optimal that the polarities of thephase encode pulses in the dummy shots are opposed to the polarities ofthe phase encode pulses for the imaging and the order of the applicationpositions (data collection positions) of the phase encode pulses in thedummy shots is opposite to the order of the application positions of thephase encode pulses for the imaging. Then, when the imaging conditionsetting unit 40 is configured to set such a pulse sequence, it ispossible to reduce the eddy current and the changes in magnetization toobtain an image with a stable image quality.

In addition, the imaging condition setting unit 40 is configured tosupply the thus generated pulse sequence to the sequence controllercontrol unit 41.

The sequence controller control unit 41 has a function of performing adrive control of the sequence controller 31 by supplying the pulsesequence acquired from the imaging condition setting unit 40 to thesequence controller 31 on the basis of the imaging start instructionfrom the input unit 33 and a function of receiving the motion signalsfrom the sequence controller 31 to supply the motion signals to themotion signal acquisition unit 42 as well as receiving raw data for theimaging from the sequence controller 31 to supply the raw data to theimaging signal acquisition unit 43.

The motion signal acquisition unit 42 has a function of acquiring themotion signals from the sequence controller control unit 41 and afunction of arranging the thus acquired motion signals in a k space (awave number space or a frequency space) provided to the motion signalacquisition unit 42.

The imaging signal acquisition unit 43 has a function of acquiring theraw data for the imaging from the sequence controller control unit 41 asthe imaging signal and a function of arranging the thus acquired imagingsignal in the k space provided to the imaging signal acquisition unit43.

FIG. 8 is a diagram showing an example of motion signals arranged in thek-space in the motion detection signal acquiring unit 42 shown in FIG.2. FIG. 9 is a diagram showing an example of imaging signals arranged inthe k-space in the imaging signal acquiring unit 43 shown in FIG. 2.

The horizontal axis of each of FIG. 8 and FIG. 9 represents the phaseencode direction Kpe in the k space and the vertical axis thereofrepresents the slice encode direction Kse. In addition, a squire mark inFIG. 8 represents a motion signal Dm and a circle mark in FIG. 9represents an imaging signal Di. As shown in FIG. 8, in the dummy shots,the encode pulse is also applied to collect the motion signal Dm.

As has been described above, the motion signal Dm is collected at everyheart beat and is data to be updated. FIG. 8 shows an example of themotion signal Dm obtained by applying the encode pulses to the two axialdirections of the phase encode direction and the slice encode direction.As has been described above the motion signal Dm may be collected byapplying the encode pulse in one axis direction. The motion signal Dm iscollected at the part A and the part C in case of using the pulsesequence shown in FIG. 5.

Also, the imaging signal Di shown in FIG. 9 is collected at the part Bin case of using the pulse sequence shown in FIG. 5. As the imagingsignals Di are collected through the segment imaging, the imagingsignals Di are partially obtained by one sequence in one heart beat.Then, the plural times of data collection by plural sequences performedover a plurality of heart beats realizes the collection of all theimaging signals Di.

In addition, as has been described above, the amount of encode for themotion signals Dm, to be more precise, the number of steps is setsmaller than the number of steps for the imaging signals Di. On theother hand, the amount of step Sm for the motion signals Dm may be thesame as or different from the amount of step Si for the imaging signalsDi. It should be noted that in order to detect the motion of largerorgans with a high precision, the motion signals Dm in the vicinity ofthe center of the k space are collected with priority.

It should be noted that, when the motion signals Dm and the imagingsignals Di are collected so as to suppress the vibration of themagnetization and its influence, the position of the last collectedmotion signal Dm by the dummy shot on the k space is set close to thecenter of the k space and the position of the following first imagingsignal Di for the imaging on the k space is set close to the position ink space of the last collected motion signal Dm by the dummy shot as muchas possible. In addition, the collecting direction of the motion signalsDm and the collecting direction of the imaging signals Di are setopposite to each other.

A motion signal component separation unit 44 has a function of readingthe motion signals from the motion signal acquisition unit 42 andseparating from each other a signal composition in which a motion isgenerally reflected and a signal composition in which the motion is notreflected and a function of supplying the signal composition in whichthe motion is reflected (motion reflected component), which is obtainedthrough the separation, to the motion amount determine unit 45. Themotion signal component separation unit 44 is configured to performreconstruction on the motion signals through a known reconstructionmethod (Fourier transform) for the separation of the motion signals andobtain profile data on a specific space position (in the encodedirection).

FIG. 10 is a diagram showing an area of the motion signals separated bythe motion signal component separating unit 44 shown in FIG. 2.

As shown in FIG. 10, for example, the motion signals are collected whilethe sagittal surface including the heart is taken as the excitationslab, the right and left direction is set as the phase encode (PE)direction, the up and down direction is set as the slice encode (SE)direction, and the body axis direction of the object P which isperpendicular to the paper surface is set as the readout direction. Thatis, as has been described above, in order to reduce the data collectiontime, the body axis direction where the amount of motion is relativelylarge is set as the readout direction. Therefore, the excitation slabmay be the coronal surface.

When the motion signals are collected while 4×4 of the encode pulses areadded in the PE direction and the SE direction and the collected motionsignals are reconstructed, the profiles in the body axis directiondivided in the areas of 4×4 can be obtained. Then, it is possible toselect only a profile showing the breathing motion in an area in thevicinity of the heart (shaded area) among the profiles as the motionreflected component.

One example of a method of selecting the profile showing the motion is amethod of selecting a profile of a part in which distortion amount ofthe reconstructed image data is considerably large. In this case, allthe profiles are compared with each other and a profile having thechange in the signal intensity exceeding a previously set threshold canbe set as the motion reflected component. It should be noted that it ispossible to easily select the profile showing the motion with such amethod in which an area expected to have the breathing motionempirically is designated in advance and merely a profile of thedesignated area is set as the motion reflected component.

FIG. 11 is a diagram showing an example of profile corresponding to thearea near the heart shown in FIG. 10.

In FIG. 11, the horizontal axis represents a position in the readoutdirection (body axis direction) and the horizontal axis represents thesignal intensity of the motion signal. Then, the solid line in FIG. 11is a profile selected as the motion reflected component. The collectionof the motion signals is executed each time the segment imagingdescribed above is advanced and therefore the motion reflected componentshown in FIG. 11 is also acquired each time the segment imagingproceeds. Then, as the segment imaging is the electrocardiographsynchronous imaging, the motion reflected component can be regarded asintermittently acquired information on the breathing motion.

Then, the motion reflected component extracted by the motion signalcomponent separation unit 44 is supplied to the motion amount determineunit 45.

The motion amount determine unit 45 has a function of determining theamount of motion on the basis of the motion reflected component receivedfrom the motion signal component separation unit 44 and a function ofsupplying the determined amount of motion to the motion correcting unit46.

FIG. 12 is a diagram explaining a method for determining a motion amountby the motion amount determine unit 45 shown in FIG. 2.

As shown in the upper part of FIG. 12, the data collection for creatingthe reference profile of the motion reflected component is performed inadvance. In the data collection for creating the reference profile, insynchronism with the ECG signal shown in FIG. 12( a), on the basis ofthe pulse sequence having the part A, the part B and the part C shown inFIG. 12( b) as the element parts, the motion signals serving as thereference are collected together with the data collection for theimaging. Then, the motion signal component separation unit 44 sets aprofile of the motion reflected component shown in FIG. 12( c) as thereference profile among the collected motion signals.

Then, at the time of the imaging, in synchronism with the ECG signalshown in FIG. 12( a), on the basis of the pulse sequence having the partA, the part B and the part C shown in FIG. 12( b) as element parts, themotion signals and the data for the imaging are collected over aplurality of times. Then, a profile #N of the n-th collected motionreflected component shown in FIG. 12( c) and a profile #N+1 of the(N+1)-th collected motion reflected component are sequentially obtainedby the motion signal component separation unit 44.

The reference profile of the motion reflected component, the profile #Nand the profile #N+1 are supplied from the motion signal componentseparation unit 44 to the motion amount determine unit 45. As a result,the motion amount determine unit 45 obtains a cross-correlation spectrumbetween a differential value of the reference profile shown in FIG. 12(d) and a differential value of the profile #N and a cross-correlationspectrum between a differential value of the reference profile and adifferential value of the profile #N+1. Then, the motion amountdetermine unit 45 detects the peak position of each of thecross-correlation spectra and determines the detected relative amountsΔd (N) and Δd (N+1) of positional shifts at the respective peakpositions as the amounts of motion at the N-th data collection and theN+1-th data collection.

When the amounts of motion Δd (N) and Δd (N+1) respectively determinedby the motion amount determine unit 45 are represented on data (dottedline) indicating the breath level shown in FIG. 12( e), the amounts areshown like circle marks. Then, such a configuration is adopted that theamounts of motion determined in this manner are supplied from the motionamount determine unit 45 to the motion correcting unit 46.

The motion correcting unit 46 has a function of acquiring the amount ofmotion from the motion amount determine unit 45 and also acquiring theimaging signal from the imaging signal acquisition unit 43 to correctthe imaging signal on the basis of the amount of motion and a functionof supplying the imaging signal after the correction to the imagereconstructing unit 47.

FIG. 13 is a diagram explaining a method for correcting imaging signalsby the motion correcting unit 46 shown in FIG. 2.

First of all, a description will be given to a case in which theparallel motion of the target area in the readout direction iscorrected. As shown in FIG. 13( a), the data collection by the pulsesequence having the part A, the part B and the part C shown in FIG. 13(b) is performed while the ECG signal is used in synchronism with thecardiac electrogram. In the part B, the imaging signals are collected byway of the segment imaging, and as shown in FIG. 13( c) and aresubjected to mapping to the k space formed in the imaging signalacquisition unit 43. By the one time segment imaging in the part B, forexample, as shown in FIG. 13(c), the imaging signals surrounded by thedotted line are collected to be arranged at predetermined positions inthe k space.

Then, the distribution of the imaging signals collected through acertain segment imaging in the readout direction and the phase encodedirection is such a distribution shown in FIG. 13( d).

On the other hand, when the amount of motion calculated from the motionsignals collected at the substantially same time as the imaging signalsis Δd, the motion correcting unit 46 creates a phase shift functionshown by the solid line of FIG. 13( e). In FIG. 13( e), the verticalaxis represents a phase and the horizontal axis represents a positionKro in the readout direction. That is, the motion correcting unit 46creates the phase shift function having a first order distribution inwhich the phase in the readout direction is proportional to the amountof motion Δd.

Then, the motion correcting unit 46 multiplies the imaging signals inthe readout direction by the phase shift function. It should be notedthat when the motion signal is the motion signal for the referenceprofile, it may be accepted to assume that the amount of motion Δd is 0.This calculation can be represented as such a conversion as shown inExpression (1) when the imaging signal in the position (Kro, Kpe, Kse)on the k space is S(Kro, Kpe, Kse).S(Kro, Kpe, Kse)→S(Kro, Kpe, Kse)·exp(−Δd·Kro)  (1)

In this way, the motion correction can be performed on the imagingsignal in terms of the parallel motion in the readout direction.Furthermore, the correction can also be performed on the imaging signalin terms of not only the parallel movement but also the linear expansionand contraction.

When the linear motion of a certain point z along with the expansion andcontraction in the readout direction is described as Expression (2-1)with use of a first order coefficient α and the zero-th ordercoefficient β, the motion of the imaging signal S on the k space isrepresented by Expression (2-2).Z→(1+α)Z+β  (2-1)S(Kro, Kpe, Kse)→S((1+α)Kro, Kpe, Kse)·exp(−βKro)  (2-2)

From Expression (2-2), it is understood that when the linear expansionand contraction motion exists, the sampling position in the readoutdirection is moved by a given ratio α as compared with a case in whichthere is no motion due to the expansion and contraction. Thus, thecorrection on the expansion and contraction is accordingly performed byobtaining the imaging signal S(Kro, Kpe, Kse) in the original samplingposition (Kro, Kpe, Kse) from the imaging signal S((1+α)Kro, Kpe, Kse)which is moved by the given ratio α. The imaging signal S(Kro, Kpe, Kse)in the original sampling position (Kro, Kpe, Kse) can be found outthrough the interpolating process in which the imaging signalS((1+α)Kro, Kpe, Kse) which is moved by the given ratio α is used.

Then, the values of α and β in Expression (2-1) can be determined fromthe first order linear deformation motion model in which the values of αand β change depending on the amount of motion Δd. The coefficient ofthe motion model can be obtained through actual measurement (actualimaging). In the imaging condition setting screen shown in FIG. 6, thedetermined values of α and β can be arbitrarily set as the first ordercoefficient and the zero-th order coefficient of the first orderexpression representing the linear deformation by way of the operationof the Linear (SI) button for instructing the motion correction inaccordance with the motion model of the linear deformation along withthe expansion and contraction.

Then, not only the correction on the expansion and contraction but alsothe correction on the parallel movement on the basis of the value of βcan be executed by the phase calculation shown in Expression (1). Inthis way, the correction on the expansion and contraction and theparallel movement due to motion can be conducted with respect to theimaging signal. Then, the imaging signal after the correction issupplied from the motion correcting unit 46 to the image reconstructingunit 47.

The image reconstructing unit 47 has a function of creating the imagedata of the object P that is the real spatial data by performing theimage reconstruction processing such as the two dimensional or threedimensional Fourier transform processing on the imaging signal after thecorrection received from the motion correcting unit 46 and a function ofperforming a necessary image processing on the created image data to besupplied to the display unit 34. Examples of the image processinginclude an MIP (Maximum Intensity Projection) processing.

Next, a description will be made of the operation and action of amagnetic resonance imaging apparatus 20.

FIG. 14 is a flowchart showing a procedure for imaging a vascular imageof the heart of the object P with the magnetic resonance imagingapparatus 20 shown in FIG. 1. The symbols including S with a number inFIG. 14 indicate each step of the flowchart.

In Step S1, the motion signals are acquired from the area that is thedetection target of the amount of motion included in the imaging slab.In addition, in Step S2 performed nearly at the same time as Step S1,the NMR signals for the imaging (the imaging signals) are acquired fromthe imaging slab including the target area for the imaging. Thisacquisition of the imaging signals is executed at a timing extremelyclose in terms of time to the acquisition of the motion signals.

For that reason, when the imaging condition setting unit 40 previouslysupplies the screen information to the display unit 34, the imagingcondition setting screen shown in FIG. 6 is displayed on the displayunit 34. When the user turns on the motion correction function throughthe operation on the input unit 33 and sets various correctionconditions, the imaging condition setting unit 40 sets an excitationslab shown in FIG. 3 on the basis of the set imaging condition andgenerates the pulse sequence shown in FIG. 4 and FIG. 5 or FIG. 7. Then,the imaging condition setting unit 40 supplies the thus generated pulsesequence to the sequence controller control unit 41.

The sequence controller control unit 41 supplies, on the basis of theimaging start instruction from the input unit 33, the pulse sequenceacquired from the imaging condition setting unit 40 to the sequencecontroller 31. As a result, the sequence controller 31 supplies, on thebasis of the pulse sequence, the control pulses to the gradient powersupply 27, the transmitter 29 and the receiver 30 respectively.Therefore, currents are supplied from the X-axis gradient power supply27 x, the Y-axis gradient power supply 27 y and the Z-axis gradientpower supply 27 z of the gradient power supply 27 to the X-axis gradientcoil 23 x, the y-axis gradient coil 23 y and the Z-axis gradient coil 23z respectively, whereby a gradient magnetic field Gx in the X-axisdirection, a gradient magnetic field Gy in the Y-axis direction and agradient magnetic field Gz in the Z-axis direction are formed in theimaging area. In addition, the RF signal is supplied from thetransmitter 29 to the RF coil 24 and the RF coil 24 transmits the RFsignal to the object P.

Then, the NMR signal generated by a nuclear magnetic resonance of anuclear spin inside the object P is received by the RF coil 24 to besupplied to the receiver 30. The receiver 30 generates raw data througha predetermined signal processing such as the detection and A/Dconversion of the NMR signal and supplies the thus generated raw data tothe sequence controller 31.

Herein, the sequence for the collection of the motion signals is set inthe pulse sequence prior to the data collection for the imaging, andtherefore not only the raw data used as the imaging signal but also themotion signal are supplied to the sequence controller 31. The sequencecontroller 31 supplies the collected imaging signal and the motionsignal to the sequence controller control unit 41.

Then, the motion signal acquisition unit 42 acquires the motion signalsfrom the sequence controller control unit 41 and arranges the acquiredmotion signals in the k space formed within the motion signalacquisition unit 42. In addition, the imaging signal acquisition unit 43acquires the image signals from the sequence controller control unit 41and arranges the acquired imaging signals in the k space formed withinthe imaging signal acquisition unit 43.

It should be noted that the collection of the imaging signals and themotion signals is performed under the electrocardiograph synchronism onthe basis of the ECG signals acquired by the ECG unit 38 and the pulsesequence.

Next, in Step S3, the motion signal component separation unit 44 readsthe motion signals from the motion signal acquisition unit 42 andseparates the motion signals into the signal component in which themotion is generally reflected and the signal component in which themotion is not reflected. For that reason, the motion signal componentseparation unit 44 executes the image reconstruction processing of themotion signals and obtains the profile data in the encode direction inthe plurality of divided areas of which number is depending to encodingamount shown in FIG. 10. Then, the motion signal component separationunit 44 compares, for example, the profile data to each other in all theareas, and separates a profile having the largest change as the motionreflected component. The motion reflected component obtained by themotion signal component separation unit 44 shown in FIG. 11 is suppliedto the motion amount determine unit 45.

Next, in Step S4, the motion amount determine unit 45 determines theamount of motion on the basis of the motion reflected component receivedfrom the motion signal component separation unit 44. To be morespecific, through a procedure shown in FIG. 12, a reference profile isdetermined and the cross-correlation spectrum between a differentialvalue of the reference profile and a differential value of the profileat the timing corresponding to the calculation target of the amount ofmotion. Then, the amount of positional shift in the peak of thecross-correlation spectrum is calculated as the amount of motion. Themotion amount determine unit 45 then supplies the thus determined amountof motion to the motion correcting unit 46.

Next, in Step S5, the motion correcting unit 46 obtains the amount ofmotion from the motion amount determine unit 45 and also obtains theimaging signals from the imaging signal acquisition unit 43 to correctthe imaging signals on the basis of the amount of motion. For example,through a procedure shown in FIG. 13, the motion correcting unit 46multiplies the phase shift function having the first order distributionin which the phase in the readout direction is in proportion to theamount of motion Δd and the imaging signals in the readout direction.That is, with the conversion shown in Expression (1), the motioncorrecting unit 46 executes the motion correction on the parallelmovement with respect to the imaging signals.

Also, for example, in the imaging condition setting screen shown in FIG.6, when the Linear (SI) button is clicked and the correction on theexpansion and contraction is also instructed, with the use of thezero-th order coefficient β and the first order coefficient α of thefirst order expression representing the linear motion model, thecorrection on the expansion and contraction is also executed in additionto that on the parallel movement. The correction on the expansion andcontraction with respect to the imaging signals can be performed throughthe interpolating processing while using the first order coefficient αon the basis of Expression (2-2).

Then, the motion correcting unit 46 supplies the imaging signals afterthe motion correction to the image reconstructing unit 47.

Next, in Step S6, the image reconstructing unit 47 performs the imagereconstruction processing such as the two dimensional or threedimensional Fourier transform processing on the imaging signals afterthe correction which is received from the motion correcting unit 46 tocreate the image data of the object P that is the real spatial data. Inaddition, the image reconstructing unit 47 performs the image processingsuch as the MIP processing on the thus created image data to be suppliedto the display unit 34. As a result, the image of the object P obtainedby performing the motion correction with use of the motion signalsacquired in the dummy shots is displayed on the display unit 34.

According to the magnetic resonance imaging apparatus 20 having theabove-mentioned configuration, the motion signals are acquired in thedummy shots set so as to be close in terms of time to the sequence forthe imaging, and therefore it can be considered that the detectiontiming for the motion signals and the timing for the collection of thedata for the imaging are nearly simultaneous to each other. In addition,according to the magnetic resonance imaging apparatus 20, it is possibleto directly acquire the motion signals by exciting the slab includingorgans such as the heart that are the collection targets of the motionsignals.

In addition, according to the magnetic resonance imaging apparatus 20,other than the data for the imaging, the motion signals enough for thenecessary range for the measurement of the amount of motion arecollected at every heart beat, and it is thus possible to collect themotion signals from the same area at different timings. Therefore, inthe magnetic resonance imaging apparatus 20, the amount of motion can bedetected with a higher precision by using the motion signals from thesame area and the detected amount of motion is used for executing themotion correction on the imaging signals. Then, with the magneticresonance imaging apparatus 20, a satisfactory image in which theoccurrence of the unsharpness or ghost is suppressed can be obtained.

Furthermore, in the magnetic resonance imaging apparatus 20, the motionsignals can be collected for each of the plurality of areas at differenttimings, and thus the signal components that are reflected by the motionare selectively extracted from the detected motion signals and can beused for the motion correction. As a result, the influence from the areawithout motion is suppressed thereby making it possible to obtain theimage to which the motion correction with a higher precision has beenconducted. In addition, it is also possible to suppress the increase inthe amount of processing associated with the motion correction.

Moreover, when the motion signals are collected with encoding in the twoaxial directions in the dummy shots, the spatial motion information canbe acquired and also when the direction in which the motion is large isset as the readout direction, it is possible to obtain the motioninformation with the less amount of data collection at a high accuracy.

Next, a description will be given of a modification example of themotion signals acquired by the motion signal acquisition unit 42.

FIG. 15 is a diagram explaining a modification example of method forarranging motion signals in the k-space formed in the motion detectionsignal acquiring unit 42 shown in FIG. 2.

As shown in FIG. 15, a part of the motion signals acquired by the motionsignal acquisition unit 42 can be substituted by the imaging signals. Inother words, the motion signal acquisition unit 42 acquires a part ofthe imaging signals and the thus acquired part of the imaging signalscan be used as the motion signals.

A square mark in FIG. 15 represents the motion signal obtained in thedummy shot and a circle mark represents an imaging signal used insteadof the motion signal. The motion signal from a part of close to thecenter of the k space has a small space frequency and is a motion signalfrom a large construction, so a high accuracy is required in many cases.In view of the above, the motion signals near the center of the k spaceare acquired by the dummy shots and the imaging signals are used for therest part.

The dummy shot is applied at each heart beat (between the adjacent Rwaves), and thus the motion signals are updated at every heart beat. Onthe other hand, the substituted imaging signals are collected throughthe segment imaging and therefore the imaging signals are graduallyobtained along with the progress in the segment imaging. Thus, thesubstituted imaging signal has always the fixed value and a part of themotion signals is not updated. However, in the case where the imagingsignals are used for the motion signals at an area away from the kspace, merely the motion signals from a small construction is notundated. Thus, when the detection of the motion signals from the largeconstruction is requested, it is considerable that even if the motionsignals from the small construction is not updated, the influence fromthis situation can be ignored in some cases.

On the other hand, a case in which there is an area where the data onthe motion signal does not exist is equivalent to a case in which a maskprocessing is performed on the k space data. Therefore, if the image isreconstructed in a state where the number of the data on the motionsignals is small, there is a risk in that information from an objectincluded in other nearby pixel may interfere as an artifact. In view ofthe above, when the imaging signals are used for the motion signals, ascompared with the case where the data on the motion signal does notexist, the generation of the artifact is suppressed and it is possibleto find out the amount of motion with a high precision.

In this way, when the motion signal acquisition unit 42 is configured toacquire the imaging signals from the sequence controller control unit 41to substitute the imaging signals for a part of the motion signals, theperiod of time required for the collection of the motion signals can bepractically reduced and it is possible to improve the time efficiencyfor the collection of the motion signals. Therefore, the flexibility inthe pulse sequence design in the segment imaging can be improved.

It should be noted that when the detection of the motion signals fromthe small construction is requested, the motion signals from a partbeing away from the center of k space in accordance with the size of theconstruction are collected by the dummy shots, and the imaging signalsmay be substituted for the motion signals in other parts. That is, whenthe motion signals only from a particular part of the k space arecollected by the dummy shots and the motion signals are updated at everyheart beat, it is possible to selectively collect the motion signalsfrom the contraction having a desired size for a short period of time tofind out the amount of motion.

The motion correction function of using the imaging signals for the partof the motion signals in the above-mentioned manner can be selected asthe correction mode. For example, a button for switching On/Off of themotion correction function associated with the substitution of theimaging signals is displayed on the imaging condition setting screendisplayed on the display unit 34, whereby the user can switch thecorrection modes arbitrarily.

Next, a description will be given of a modification example of theseparation method for the motion signals by the motion signal componentseparation unit 44.

FIG. 16 is a flowchart explaining a modification example of method forseparating motion signals by the motion signal component separating unit44 shown in FIG. 2. The symbols including S with a number in FIG. 16indicate each step of the flowchart. FIG. 17 is a diagram showing anexample of area formed by the image reconstruction processing shown inFIG. 16. FIG. 18 is a conceptual diagram of the motion part mask shownin FIG. 16.

In Step S10 of FIG. 16, as described above, the motion signal componentseparation unit 44 acquires the motion signals arranged in the k spacefrom the motion signal acquisition unit 42 to execute the imagereconstruction processing on the motion signals. As a result, as shownin FIG. 17, it is possible to obtain the profile data in the readoutdirection in the areas of which the number is in accordance with theamount of encodes divided in the PE direction and the SE direction.

Next, in Step S11, the motion signal component separation unit 44creates a motion part mask having a weight in accordance with the sizeof the motion on the basis of the profile data at each area. That is, asshown in FIG. 16, the motion part mask having different weight in one orboth of the PE direction and the SE direction is created by the motionsignal component separation unit 44. FIG. 16 shows an example of themotion part mask having a different weight only in the PE direction buta different weight may be set in the SE direction.

The weight of the motion part mask is set large in a pixel having anarea with a large motion (the heart in this example), and on the otherhand the weight is set small in a pixel having an area with a smallmotion or no signal (the chest wall or the like in this example). Thesize of the motion in each pixel can be obtained by referring to each ofthe profile data.

Next, in Step S12, the motion signal component separation unit 44applies the motion part mask on the image reconstruction data obtainedthrough the image reconstruction processing on the motion signals toexecute a weighing processing with respect to the reconstruction data ofthe motion signals in the PE direction and the SE direction.

Next, in Step S13, the motion signal component separation unit 44applies an inverse reconstruction processing on the reconstruction dataafter the weighing processing only in the PE direction and the SEdirection.

Next, in Step S14, the motion signal component separation unit 44 addsthe data, that is turned into the k space data only in the PE directionand the SE direction, in the PE direction and the SE direction.

As a result, it is possible to obtain the profile data of the motionreflected component in the readout direction shown in FIG. 11. This isequivalent to the case where the signal component in which the motion isreflected is separated. In this way, when the signal component in whichthe motion is reflected is obtained through the signal additionprocessing associated with the weighing in the encode direction, the SNratio of the measurement for the motion reflected component can be madelarge and the detection accuracy for the amount of motion can beimproved.

It should be noted that the separation method for the motion signals canalso be selected by the user as the correction mode.

FIG. 19 is a block diagram showing a magnetic resonance imagingapparatus according to a second embodiment of the present invention.

In a magnetic resonance imaging apparatus 20A shown in FIG. 19, afunction of the imaging condition setting unit 40 and a point that aprofile producing unit 50 instead of the motion signal componentseparating unit 44 is included are different from those of the magneticresonance imaging apparatus 20 shown in FIG. 1. Other constructions andoperations of the magnetic resonance imaging apparatus 20A are notdifferent from those of the magnetic resonance imaging apparatus 20shown in FIG. 1 substantially. Therefore, only a functional blockdiagram of the computer 20 is shown, attaching same number to a samecomponent as that of the magnetic resonance imaging apparatus 20 andomitting explanation thereof.

The imaging condition setting unit 40 of the magnetic resonance imagingapparatus 20A is provided with a function of setting the pulse sequencecapable of collecting the motion signals including the component inwhich the motion is reflected as main component in the dummy shotsapplied at timing sufficiently close to the data collection for theimaging.

FIG. 20 is a diagram showing an example of pulse sequence produced bythe imaging condition setting unit 40 of the magnetic resonance imagingapparatus 20A shown in FIG. 19.

As shown in FIG. 20, the imaging condition setting unit 40 creates, forexample, the part C that is the sequence for the dummy shot followingthe part B that is the sequence for the imaging. The part B is asequence for the segment imaging for applying the RF signal in anextremely short repetition time TR while sequentially changing the phaseencode pulse PE (B) for collecting the three dimensional imaging signalACQ (B). In the part B, the slice direction is set as the sagittalsurface direction including the heart.

Then, the part C is a sequence for collecting the motion signals havingthe component in which the motion is reflected as the main component.For that reason, the slice encode direction of the part C is set as adifferent direction from the slice encode direction of the part B and inthe part C, the slice direction is set as the coronal surface directionincluding the heart. Furthermore, the part C is a sequence for applyinga spoiler pulse before and after a slice gradient magnetic field pulsePc. Then, the motion signal ACQ (c) having the component in which themotion is reflected by the thus constructed the part C as the maincomponent is collected.

FIG. 21 is a diagram showing a slab position excited by the pulsesequence shown in FIG. 20.

As shown in FIG. 21, the slab B represented by the solid line with aslice gradient magnetic field pulse Gss in the part B of the pulsesequence shown in FIG. 20 is excited. Subsequently, the slab Crepresented by the dotted line with the slice gradient magnetic fieldpulse Pc of the part C is excited. Then, in the part C, the motionsignal ACQ (C) is collected from the rectangular area surrounded by thedotted line and the solid line excited by the slice gradient magneticfield pulses Gss and Pc of both the part B and the part C.

Then, by controlling the slice gradient magnetic field pulse Pc of thepart C and adjusting the position of the excited slab C, it is possiblethat the area having the motion such as the heart is selectivelyincluded in the rectangular area to the area without any motion. In thisway when the slice gradient magnetic field pulse Pc of the part C iscontrolled such that the area having the motion is included in therectangular area, it is possible to collect the motion signal ACQ (C)having the component in which the motion is reflected as the maincomponent. As to the position of the slab C, there are a method ofobtaining the position from the experimental data, a method ofestimating the position on the basis of the previously collected motioninformation at each position, and the like.

On the other hand, the profile producing unit 50 has a function ofacquiring the motion signals obtained through the data collection on thebasis of the pulse sequence set by the imaging condition setting unit 40from the motion signal acquisition unit 42 and a function of obtainingthe profile data in the readout direction by performing the imagereconstruction processing on the thus acquired motion signals.Furthermore, the profile producing unit 50 is configured to supply thethus obtained profile data to the motion amount determine unit 45 as themotion reflected component.

Then, in the magnetic resonance imaging apparatus 20A, the pulsesequence shown in FIG. 20 is created by the imaging condition settingunit 40 such that the slab including the area having the motion such asthe heart is excited by the sequence for collecting the motion signals.Then, the motion signals collected following the collection of theimaging signals are supplied to the profile producing unit 50. Themotion signals supplied to the profile producing unit 50 have thecomponent in which the motion is reflected as the main component. Inview of the above, the profile producing unit 50 performs the imagereconstruction processing on the motion signals to obtain the profiledata in the readout direction. Then, the profile producing unit 50supplies the thus obtained profile data to the motion amount determineunit 45 as the motion reflected component. Furthermore, as in the caseof the magnetic resonance imaging apparatus 20 shown in FIG. 1, themotion correction is executed on the imaging signals, thereby making itpossible to obtain the image after the motion correction.

A part of elements and functions of the magnetic resonance imagingapparatuses 20 and 20A in each of the embodiments may be omitted andconversely the functions of the magnetic resonance imaging apparatuses20 and 20A may be combined with each other. For example, such aconfiguration may be adopted that the motion signal component separationunit 44 of the magnetic resonance imaging apparatus 20 shown in FIG. 1may be omitted and the profiles of all the motion signals are utilizedas the motion reflected components to perform the motion correction. Inaddition, such a configuration may also be adopted that the magneticresonance imaging apparatus 20A shown in FIG. 19 is provided with themotion signal component separation unit 44 and further the component inwhich the motion is reflected from is separated from the motion signalseach having the component in which the motion is reflected as the maincomponent.

In addition, in the above-mentioned magnetic resonance imagingapparatuses 20 and 20A, the example in which the correction processingon the imaging signals is performed on the basis of the amount of motionhas been described, but the magnetic resonance imaging apparatuses 20and 20A may be configured so as not to perform the correction processingand so as to select the imaging signals in a particular range for theimage reconstruction. In this case, for example, the computer 32 isprovided with an image signal selecting unit instead of the motioncorrecting unit 46.

The image signal selecting unit selects the imaging signals in theparticular range among the thus acquired imaging signals from theimaging signal acquisition unit 43 on the basis of the amount of motiondetermined in the motion amount determine unit 45. As an example of theselection method for the imaging signals, there is one includingcreating a gating window in accordance with the amount of motion andselecting the imaging signals in the particular range from the imagingsignals by using the gating window. For example, the gating window iscreated such that the imaging signals in a range where the amount ofmotion does not exceed a threshold are selected. With the thus createdgating window, it is possible to remove the imaging signals showing thelarge motion from the imaging signals for the image reconstruction.

Then, the image reconstructing unit 47 uses the imaging signals selectedby the image signal selecting unit to reconstruct an image. In this way,through the sorting out of the imaging signals, it is possible to obtainan image to which the influence of the motion is small and which is evencloser to the reference image.

1. A magnetic resonance imaging apparatus comprising: an imaging signalacquisition unit configured to acquire magnetic resonance signals froman object as imaging signals; a motion signal acquisition unitconfigured to repetitively acquire magnetic resonance signals having aphase encoding amount less than that used for imaging signals as motionsignals; a motion amount determination unit configured to obtain amotion amount using the motion signals; a selection unit configured toselect imaging signals within a specific range in accordance with themotion amount; a motion correction unit configured to perform correctionprocessing of the imaging signals or the selected imaging signals withinthe specific range in accordance with the motion amount; and an imagereconstruction unit configured to reconstruct an image using theselected imaging signals within the specific range after the correctionprocessing.
 2. A magnetic resonance imaging apparatus according to claim1, further comprising: a motion reflected component acquisition unitconfigured to acquire signal component reflecting a motion from themotion signals, wherein said motion amount determine unit is configuredto obtain the motion amount from the signal component reflecting themotion.
 3. A magnetic resonance imaging apparatus according to claim 1,wherein: said motion amount determine unit is configured to obtain themotion amount using motion signals acquired from a same part at amutually different timings.
 4. A magnetic resonance imaging apparatusaccording to claim 1, wherein: said motion signal acquisition unit isconfigured to acquire the motion signals with encoding in two axisdirections.
 5. A magnetic resonance imaging apparatus to claim 1wherein: said motion signal acquisition unit is configured to acquirethe motion signals from a same slab as an imaging slab for acquiring theimaging signals.
 6. A magnetic resonance imaging apparatus according toclaim 1, wherein: said motion signal acquisition unit is configured toacquire the motion signals by assigning a body axis direction of theobject to a readout direction.
 7. A magnetic resonance imaging apparatusaccording to claim 1, wherein: said motion signal acquisition unit isconfigured to acquire the motion signals at least one of before andafter acquisition of the imaging signals.
 8. A magnetic resonanceimaging apparatus according to claim 1, further comprising: an imagingcondition setting unit configured to set an imaging condition foracquiring the motion signals, the imaging condition including the phaseencoding amount.
 9. A magnetic resonance apparatus according to claim 1,wherein: said motion signal acquisition unit is configured to set asequence for acquiring the motion signals so as to decrease a stepamount of an encoding pulse for acquiring the motion signals gradually,and said imaging signal acquisition unit is configured to set a sequencefor acquiring the imaging signals so as to increase a step amount of anencoding pulse for acquiring the imaging signals gradually.
 10. Amagnetic resonance imaging apparatus according to claim 1, wherein: saidmotion signal acquisition unit is configured to set a slab for acquiringthe motion signals so as to include a part serving as a detection targetof the motion amount in a rectangular area formed by the slab and animaging slab for acquiring the imaging signals to acquire the motionsignals from the rectangular area with a sequence for acquiring themotion signals subsequent to a sequence for acquiring the imagingsignals, the slab being set in a direction different from the imagingslab.
 11. A magnetic resonance imaging apparatus according to claim 1,wherein: said motion correction unit configured to perform thecorrection processing with linear expansion and contraction correction.12. A magnetic resonance imaging apparatus according to claim 2,wherein: said motion reflected component acquisition unit is configuredto produce a reconstructed data on an area by reconstructing the motionsignals to obtain a profile of reconstructed data on at least one of theareas as the signal component reflecting the motion, the one showing abreath motion.
 13. A magnetic resonance imaging apparatus according toclaim 2, wherein: said motion reflected component acquisition unit isconfigured to produce a reconstructed data on areas by reconstructingthe motion signals to obtain a profile of reconstructed data obtained byweighting the reconstructed data on the areas respectively andsubsequently adding processed data in an encode direction as the signalcomponent reflecting the motion, the processed data being subjected toinverse reconstruction processing in the encode direction.